The Time Division Duplex (TDD) system divides uplink (sent by a terminal to a base station) and downlink (sent by a base station to a terminal) resources in the time domain, and usually performs uplink and downlink resource allocation by taking a time slot or subframe as the unit. Under normal circumstances, the base station uses a broadcast signaling to notify all terminals within a cell of the uplink and downlink resource allocation situation in the semi-static manner.
For example, FIG. 1 shows a frame structure (also called the second type of frame structure, i.e. the frame structure type 2) of a TDD mode in the Long Term Evolution (LTE) system. In the frame structure, one 10 ms (i.e. 307200 Ts, Ts is the sampling interval, and 1 Ts=1/30720000 sec) radio frame is divided into two half-frames, and the length of each half-frame is 5 milliseconds (i.e. 5 ms). Each half-frame contains five 1 ms subframes. The role of each subframe is as shown in Table, wherein D represents a downlink subframe used for transmitting the downlink signal. U represents an uplink subframe used for transmitting the uplink signal. One uplink or downlink subframe is divided into two 0.5 ms time slots. S represents a special subframe containing three special time slots, namely DwPTS (Downlink Pilot Time Slot, used for transmitting downlink signals), GP (Guard Period) and UpPTS (Uplink Pilot Time Slot, used for transmitting uplink signals).
TABLE 1LTE uplink and downlink configurationSwitchingConfig-pointSubframe numberurationperiod012345678905 msDSUUUDSUUU15 msDSUUDDSUUD25 msDSUDDDSUDD310 ms DSUUUDDDDD410 ms DSUUDDDDDD510 ms DSUDDDDDDD65 msDSUUUDSUUD
The Orthogonal Frequency Division Multiplexing (OFDM) technology is used in the downlink of the LTE system, and the Single Carrier-Frequency Division Multiple Access (SC-FDMA) technology (or called the DFT Spread-OFDM technology) is used in the uplink thereof. In the normal Cyclic Prefix (CP) condition, a time slot consists of 7 OFDM symbols or SC-FDMA symbols. In the extended CP (Extended cyclic prefix) condition, a time slot consists of 6 OFDM symbols or SC-FDMA symbols. FIG. 2 takes the normal CP for example and provides a schematic diagram of a downlink subframe. Wherein, the bandwidth of one RE (Resource Element) is 15 kHz, and occupies one OFDM or SC-FDMA symbol in the time domain. One RB (Resource Block) occupies 12 REs in the frequency domain and occupies one time slot in the time domain.
The LTE system transmits the downlink data services through the Physical Downlink Shared CHannel (PDSCH), and transmits the uplink data services through the Physical Uplink Shared CHannel (PUSCH). In addition, the LTE physical layer also contains some control channels used for assisting in the uplink and downlink data transmission. For example:
An PDCCH (Physical Downlink Control Channel) or EPDCCH (Enhanced Physical Downlink Control Channel) is used for carrying the following control information:
uplink scheduling signaling (such as a Downlink Control Information (DCI) format 0/4). The signaling is used to indicate the terminal information such as the uplink resource allocation situation, and the modulation and coding scheme of the transport block, and so on;
downlink scheduling signaling (such as a DCI format 1/1A/1B/1C/1D/2/2A/2B/2C/2D). The signaling is used to indicate the terminal information such as the downlink resource allocation situation, and the modulation and coding scheme of the transport block, and so on;
uplink power control signaling (such as a DCI format 3/3A). The signaling is used to indicate the terminal the adjustment situation of uplink transmit power
An PHICH (Physical Hybrid ARQ Indicator CHannel) is used to indicate whether the uplink data transmission result is correct.
Each downlink control message is finally transmitted in the PDCCH in the form of one or more CCEs (Control Channel Elements) after going through processes of the CRC (Cyclic Redundancy Check) addition, channel coding, and rate matching and so on. Wherein, during the CRC addition, it also needs to further use the corresponding RNTI to scramble the CRC.
The physical resources transmitted in the physical downlink control channel are the control channel elements (CCEs), the size of a CCE is nine Resource Element Groups (REGs), i.e., 36 Resource Elements (REs), and one PDCCH may occupy 1, 2, 4 or 8 CCEs. The four types of PDCCHs that occupy 1, 2, 4 and 8 CCEs use the tree Aggregation, that is, the PDCCH occupying one CCE may start from any CCE position, the PDCCH occupying two CCEs starts from an even-numbered CCE position, the PDCCH occupying four CCEs starts from a CCE position that is an integer multiple of 4, and the PDCCH occupying 8 CCEs starts from a CCE position that is an integer multiple of 8. In one radio frame, if the normal Cyclic Prefix (CP) is used, the first one to three OFDM symbols of the first time slot of each subframe can bear physical resources of the PDCCH, and the remaining symbols can bear physical resources of the Physical Downlink Shared Channel (PDSCH).
For each Aggregation Level, the standard defines a Search Space including a Common search space and UE-specific search spaces. The number of CCEs in the entire search space is determined by the number of OFDM symbols occupied by the control region indicated by the PCFICH in each downlink subframe and the number of PHICH groups. The UE performs the blind detection on all possible PDCCH code rates in the search space in accordance with the DCI format of the transmission mode.
In the LTE system, the uplink and downlink data transmission and its corresponding control signaling have a certain timing sequence relationship. The scheduling signaling of the PDSCH and the PDSCH are transmitted in the same subframe (assumed as a subframe n), the feedback signaling (ACK/NACK, used for indicating whether the data are transmitted correctly) of the PDSCH is transmitted in the subframe n+k after the PDSCH. The PUSCH is transmitted in the subframe m, the scheduling signaling of the PUSCH is transmitted in the previous subframe m−p, and the feedback signaling (ACK/NACK) is transmitted in the latter subframe m+q.
In some wireless communication scenarios, the uplink and downlink service changes in the base station serving area are very dramatic. For example, in some small cells or home environments, the number of users served by one base station is very small, the system load is relatively low, the proportions of uplink and downlink data amounts in the serving area change rapidly. Under such conditions, semi-statically allocating the uplink and downlink resources of the TDD system affects the resource allocation efficiency. Under this background, the LTE R12 release introduces the dynamic uplink and downlink configuration adjustment function to the TDD mode. But how the base station quickly notifies the terminal of the uplink and downlink configuration used by the current cell is still a problem to be solved.